#include<stdio.h>
int main(void)
{
    double N,i,t,fenzi=2,fenmu=1;
    double sum = 0.0;
    scanf("%lf",&N);

    for(i=0;i<N;i++){

        sum += 1.0*fenzi/fenmu;
        t = fenzi;
        fenzi += fenmu;
        fenmu = t;
    }

    printf("%.2lf",sum);

    return 0;
}